1. Field of the Invention
The present invention relates generally to the field of medical devices, and more particularly, to encapsulation of stents.
2. Description of Related Art
Stents and related endoluminal devices are currently used by medical practitioners to treat portions of the vascular system that become so narrowed that blood flow is restricted. Stents are tubular structures, usually of metal, which are radially expandable to hold a narrowed blood vessel in an open configuration. Such narrowing (stenosis) occurs, for example, as a result of the disease process known as arteriosclerosis. Angioplasty of a coronary artery to correct arteriosclerosis may stimulate excess tissue proliferation which then blocks (restenosis) the newly reopened vessel. While stents are most often used to xe2x80x9cprop openxe2x80x9d blood vessels, they can also be used to reinforce collapsed or narrowed tubular structures in the respiratory system, the reproductive system, biliary ducts or any other tubular body structure. However, stents are generally mesh-like so that endothelial and other cells can grow through the openings resulting in restenosis of the vessel.
Polytetrafluoroethylene (PTFE) has proven unusually advantageous as a material from which to fabricate blood vessel grafts or prostheses used to replace damaged or diseased vessels. This is partially because PTFE is extremely biocompatible causing little or no immunogenic reaction when placed within the human body. This is also because in its preferred form, expanded PTFE (ePTFE), the material is light and porous and is potentially colonized by living cells becoming a permanent part of the body. The process of making ePTFE of vascular graft grade is well known to one of ordinary skill in the art. Suffice it to say that the critical step in this process is the expansion of PTFE into ePTFE following extrusion from a paste of crystalline PTFE particles. Expansion represents a controlled longitudinal stretching in which the PTFE is stretched up to several hundred percent of its original length. During the expansion process fibrils of PTFE are drawn out of aggregated PTFE particle (nodes), thereby creating a porous structure.
If stents could be enclosed in ePTFE, cellular infiltration could be limited, hopefully preventing or limiting restenosis. Early attempts to produce a stent enshrouded with ePTFE focused around use of adhesives or physical attachment such as suturing. However, such methods are far from ideal, and suturing, in particular, is very labor intensive. More recently, methods have been developed for encapsulating a stent between two tubular ePTFE members whereby the ePTFE of one-member contacts and bonds to the ePTFE of the other member through the openings in the stent. However, such a monolithically encapsulated stent tends to be rather inflexible. In particular, radial expansion of the stent may stress and tear the ePTFE. There is a continuing need for a stent that is encapsulated to prevent cellular intrusion and to provide a smooth inner surface blood flow and yet still capable of expansion without tearing or delaminating and is relatively more flexible.
The present invention is directed to encapsulated stents wherein flexibility of the stent is retained, despite encapsulation.
It is an object of this invention to provide a stent device that has improved flexibility, yet maintains its shape upon expansion.
It is also an object of this invention to provide a stent encapsulated to prevent cellular infiltration wherein portions of the stent can move during radial expansion without stressing or tearing the encapsulating material.
These and additional objects are accomplished by an encapsulation process that leaves portions of the stent free to move during expansion without damaging the ePTFE covering. The most basic form of this invention is produced by placing a stent over an inner ePTFE member (e.g., supported on a mandrel) and then covering the outer surface of the stent with an outer ePTFE tube into which slits have been cut. The outer ePTFE tube is then laminated to the inner ePTFE through openings in the stent structure to capture the stent. By selecting the size and location of the slits it is possible to leave critical parts of the stent unencapsulated to facilitate flexibility and expansion. Not only does the slit prevent capture of the underlying PTFE, it forms a focal point for the PTFE to flex. A more complex form of the process is to place over the stent an ePTFE sleeve into which apertures have been cut. This xe2x80x9claceyxe2x80x9d outer sleeve leaves portions of the stent exposed for increased flexibility and for movement of the stent portions during expansion without damaging the ePTFE. Although a single stent can be used, these approaches lend themselves to use of a plurality of individual ring stents spaced apart along an inner ePTFE tube and covered by a xe2x80x9claceyxe2x80x9d ePTFE sleeve.
In the present invention, individual ring stents are partially encapsulated using the procedure outlined above. Preferably, ring stents of zigzag sinusoidal structure are placed xe2x80x9cin phasexe2x80x9d (e.g., peaks and valleys of one stent aligned with those of a neighboring stent) on the surface of a tubular ePTFE graft supported by a mandrel. A sleeve of ePTFE is cut using CO2 laser so that openings are created, resulting in a xe2x80x9claceyxe2x80x9d pattern. This xe2x80x9claceyxe2x80x9d sleeve is then placed over the ring stents. The resulting structure is then subjected to heat and pressure so that regions of ePTFE become laminated or fused together where the lacey sleeve contacts the tubular graft. In addition, the ends of the stent can be completely encapsulated, by known methods, to stabilize the overall structure.
A more complete understanding of the encapsulation process will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings which will first be described briefly.